Bug 1025240 - test-native-options should cleanup the preference branches that it creates

nit: now that [no-prefs/package.json](#diff-0) has a specific id (and thus a prefs branch), this test is now needlessly complicated, and my comment below doesn't even make sense anymore.

(my self-imposed limitation while porting these unit-tests from cfx was not to change `package.json` files like you did above. that's why this test used the global preferences branch, so i used events because i couldn't simply iterate over it to check that it's empty -- which is now possible)